As pistons used in swash plate type hydraulic pumps, hydraulic motors, and the like, there are male pistons and female pistons. Known as a liquid-pressure pump including a female piston is a liquid-pressure pump of PTL 1, for example.
The female piston of the liquid-pressure pump of PTL 1 includes a concave spherical surface, and a convex spherical portion of a shoe is supported by the concave spherical surface so as to be slidable and rotatable. Therefore, as with the male piston, the piston can rotate relative to the shoe around a center point of the convex spherical portion, and pressure resistance performance of the piston and the shoe can be improved.